Mode suppression in acoustic resonators

ABSTRACT

Acoustic resonators, such as bulk acoustic wave (BAW) resonators, are disclosed that include mode suppression structures. Acoustic resonators, including stacked crystal filters (SCFs), are disclosed that include spurious mode suppression by modifying a piezoelectric coupling profile within one or more layers of an SCF. Mode suppression configurations may include structures with one or more inverted polarity piezoelectric layers, one or more non-piezoelectric layers, one or more thicker electrodes of the SCF, and combinations thereof. Symmetric input and output electrical response for SCFs with mode suppression configurations may be exhibited by including piezoelectric materials with different electromechanical coupling values and/or by dividing stress profiles differently by configuring different thicknesses for input and output sides of SCFs.

FIELD OF THE DISCLOSURE

The present disclosure relates to acoustic resonators, such as bulkacoustic wave (BAW) resonators and, in particular, to mode suppressionin acoustic resonators including stacked crystal filters (SCFs).

BACKGROUND

Wireless communication networks have continued to evolve in order tokeep up with ever increasing data transmission demands of moderntechnology. With each new generation of cellular network technology,higher integration and smaller device sizes are needed to provideimproved data capacity, connectivity, and coverage. As modern mobilecommunication systems are developed with increased complexity, highperformance acoustic resonators are increasingly used as filters in suchsystems.

Acoustic filters, including bulk acoustic wave (BAW) resonators, aresometimes arranged in ladder network topologies that exhibit manydesirable features, but can also provide limited operating bandwidths.Other acoustic filter configurations such as stacked crystal filters(SCFs) and coupled resonator filters (CRFs) can provide larger operatingbandwidths. SCFs, as compared with CRFs, may typically have simplerconfigurations including fewer layers which can provide somewhat easierfabrication.

In such acoustic filters, degraded filter rejections can be caused byspurious resonances or spurious modes that are excited at certainfrequencies. Spurious modes may be addressed by making use of the finitebandwidth of reflector structures in a solidly mounted resonator (SMR)configuration. Such reflector structures can include reflector layersthat are designed to have suitable reflectivity within a desired filterbandwidth and are also designed to be lossy in filter stopbands,particularly at frequencies of spurious responses. Using this approach,many reflector layers are usually needed to obtain the desired reflectorselectivity. Moreover, in filter configurations where multiple SCFstructures with different frequencies are typically used, the spuriousresponses of the multiple SCFs tend not to overlap. As such, a singlereflector structure may not be able to suppress spurious responses ofall SCFs simultaneously. As advancements in mobile communication systemsprogress, the art continues to seek improved acoustic resonators andfilter configurations capable of overcoming such challenges.

SUMMARY

The present disclosure relates to acoustic resonators, such as bulkacoustic wave (BAW) resonators and, in particular, to mode suppressionin acoustic resonators. Acoustic resonators, including stacked crystalfilters (SCFs), are disclosed that include spurious mode suppression bymodifying a piezoelectric coupling profile within one or more layers ofan SCF. Mode suppression configurations may include structures with oneor more inverted polarity piezoelectric layers, one or morenon-piezoelectric layers, one or more thicker electrodes of the SCF, andcombinations thereof. Symmetric input and output electrical response forSCFs with mode suppression configurations may be exhibited by includingpiezoelectric materials with different electromechanical coupling valuesand/or by dividing stress profiles differently by configuring differentthicknesses for input and output sides of SCFs.

In one aspect, an acoustic resonator comprises: a first piezoelectriclayer; a second piezoelectric layer; a shared electrode between thefirst piezoelectric layer and the second piezoelectric layer; a firstelectrode on the first piezoelectric layer opposite the sharedelectrode; a second electrode on the second piezoelectric layer oppositethe shared electrode; and a third piezoelectric layer between the secondelectrode and the shared electrode, the third piezoelectric layer havinga polarity that is opposite a polarity of the second piezoelectriclayer.

In certain embodiments, the third piezoelectric layer is between thesecond electrode and the second piezoelectric layer. In certainembodiments, the third piezoelectric layer is between the sharedelectrode and the second piezoelectric layer. In certain embodiments,the second piezoelectric layer has a higher electromechanical couplingthan the first piezoelectric layer. In certain embodiments, the firstpiezoelectric layer has a higher electromechanical coupling than thesecond piezoelectric layer. In certain embodiments, the acousticresonator further comprises a fourth piezoelectric layer between thefirst electrode and the shared electrode, the fourth piezoelectric layerhaving a polarity that is opposite a polarity of the first piezoelectriclayer. In certain embodiments, the fourth piezoelectric layer is betweenthe shared electrode and the first piezoelectric layer.

In certain embodiments, the third piezoelectric layer has a differentthickness than the fourth piezoelectric layer. In certain embodiments, acombined thickness of the first piezoelectric layer and the fourthpiezoelectric layer is different than a combined thickness of the thirdpiezoelectric layer and the second piezoelectric layer. In certainembodiments, the first piezoelectric layer has a different thicknessthan the second piezoelectric layer.

In certain embodiments, the third piezoelectric layer is between thesecond electrode and the second piezoelectric layer. In certainembodiments, the fourth piezoelectric layer is between the firstelectrode and the first piezoelectric layer. In certain embodiments, thefourth piezoelectric layer is between the first piezoelectric layer andthe first electrode and the third piezoelectric layer is between thesecond electrode and the second piezoelectric layer. In certainembodiments, the first piezoelectric layer has a different thicknessthan the second piezoelectric layer.

In another aspect, an acoustic resonator comprises: a firstpiezoelectric layer; a second piezoelectric layer; a shared electrodebetween the first piezoelectric layer and the second piezoelectriclayer; a first electrode on the first piezoelectric layer opposite theshared electrode; a second electrode on the second piezoelectric layeropposite the shared electrode; and a non-piezoelectric layer between thefirst electrode and the second electrode. In certain embodiments, thenon-piezoelectric layer comprises a dielectric layer. In certainembodiments, the dielectric layer comprises silicon dioxide or siliconnitride. In certain embodiments, the non-piezoelectric layer is betweenthe second electrode and the second piezoelectric layer. In certainembodiments, the non-piezoelectric layer is between the first electrodeand the first piezoelectric layer. In certain embodiments, the sharedelectrode is a metal layer.

In another aspect, an acoustic resonator comprises: a firstpiezoelectric layer; a second piezoelectric layer; a shared electrodebetween the first piezoelectric layer and the second piezoelectriclayer; a first electrode on the first piezoelectric layer opposite theshared electrode; and a second electrode on the second piezoelectriclayer opposite the shared electrode, wherein the second electrode isthicker than the first electrode. In certain embodiments, the secondelectrode is at least fifty percent thicker than the first electrode. Incertain embodiments, the second electrode is at least two times thickerthan the first electrode.

In another aspect, any of the foregoing aspects, and/or various separateaspects and features as described herein, may be combined for additionaladvantage. Any of the various features and elements as disclosed hereinmay be combined with one or more other disclosed features and elementsunless indicated to the contrary herein.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1A is a diagram illustrating a typical stacked crystal filter(SCF).

FIG. 1B illustrates a transmission response of the SCF of FIG. 1Aexhibiting multiple modes formed along a frequency range.

FIGS. 2A-2C illustrate stress profiles for three order modes of FIG. 1B.

FIG. 3 illustrates an equivalent circuit of the SCF of FIG. 1A about thesecond order mode of FIG. 2B.

FIG. 4A illustrates the SCF of FIG. 1A with shunt inductors arranged tocompensate for shunt capacitances.

FIG. 4B illustrates the equivalent circuit of FIG. 4A.

FIG. 5 is an illustration of a slab of piezoelectric material with avarying electromechanical coupling along the z-axis.

FIG. 6 illustrates the stress profile of the third order mode asillustrated in FIG. 2C with a horizontal dashed line superimposed toseparate the second piezoelectric layer into top and bottom portions.

FIG. 7A illustrates the stress profile of the third order mode for anSCF where portions of a second piezoelectric layer are replaced by anon-piezoelectric layer for mode suppression.

FIG. 7B illustrates a stress profile of the third order mode for an SCFwhere a second electrode is arranged with increased thickness for modesuppression.

FIG. 7C illustrates the stress profile of the third order mode for anSCF where an inverted piezoelectric layer is arranged for modesuppression.

FIG. 8 illustrates a comparison plot for transmission responses of theSCF of FIG. 7C and the SCF of FIG. 1A.

FIG. 9 is a diagram illustrating an SCF that includes a secondpiezoelectric layer with a higher electromechanical coupling material ascompensation for mode suppression.

FIG. 10A is a Smith chart comparing impedance for inputs of the SCF ofFIG. 1A, the SCF of FIG. 7C, and the SCF of FIG. 9.

FIG. 10B is a Smith chart comparing impedance for outputs of the SCF ofFIG. 1A, the SCF of FIG. 7C, and the SCF of FIG. 9.

FIG. 10C illustrates a comparison plot for the transmission responses ofthe SCF of FIG. 1A, the SCF of FIG. 7C, and the SCF of FIG. 9.

FIG. 11 is a stress profile diagram for the first order mode of an SCFwhere a third piezoelectric layer having inversed polarity is arrangedbetween a first electrode and a shared electrode for mode suppression.

FIG. 12 is a diagram illustrating an SCF that includes mode suppressionfor the first order mode and the third order mode.

FIG. 13 illustrates the transmission response for the SCF of FIG. 12.

FIG. 14 is a diagram illustrating an SCF that includes mode suppressionfor the second order mode and the third order mode.

FIG. 15 illustrates the transmission response for the SCF of FIG. 14.

FIG. 16 is a diagram illustrating an SCF that includes mode suppressionfor the first order mode and the third order mode without requiringcompensation configurations.

FIG. 17A illustrates the transmission response for the SCF of FIG. 16.

FIG. 17B is a Smith chart illustrating impedance for an input of the SCFof FIG. 16.

FIG. 17C is a Smith chart comparing impedance for an output of the SCFof FIG. 16.

FIG. 18 is a diagram illustrating an SCF that includes mode suppressionfor the second order mode and the third order mode without requiringcompensation configurations.

FIG. 19A illustrates the transmission response for the SCF of FIG. 18.

FIG. 19B is a Smith chart illustrating impedance for an input of the SCFof FIG. 18.

FIG. 19C is a Smith chart comparing impedance for an output of the SCFof FIG. 18.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Embodiments are described herein with reference to schematicillustrations of embodiments of the disclosure. As such, the actualdimensions of the layers and elements can be different, and variationsfrom the shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are expected. For example, aregion illustrated or described as square or rectangular can haverounded or curved features, and regions shown as straight lines may havesome irregularity. Thus, the regions illustrated in the figures areschematic and their shapes are not intended to illustrate the preciseshape of a region of a device and are not intended to limit the scope ofthe disclosure. Common elements between figures may be shown herein withcommon element numbers and may not be subsequently re-described.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to acoustic resonators, such as bulkacoustic wave (BAW) resonators and, in particular, to mode suppressionin acoustic resonators. Acoustic resonators, including stacked crystalfilters (SCFs), are disclosed that include spurious mode suppression bymodifying a piezoelectric coupling profile within one or more layers ofan SCF. Mode suppression configurations may include structures with oneor more inverted polarity piezoelectric layers, one or morenon-piezoelectric layers, one or more thicker electrodes of the SCF, andcombinations thereof. Symmetric input and output electrical response forSCFs with mode suppression configurations may be exhibited by includingpiezoelectric materials with different electromechanical coupling valuesand/or by dividing stress profiles differently by configuring differentthicknesses for input and output sides of SCFs.

BAW resonators in stacked crystal filters (SCFs) are used in manyhigh-frequency filter applications. FIG. 1A is a diagram illustrating atypical SCF 10. The SCF 10 includes first and second piezoelectriclayers 12, 14 with a shared electrode 16 serving as a common groundplane that acoustically couples the first and second piezoelectriclayers 12, 14. A first electrode 18 is arranged on the firstpiezoelectric layer 12 and a second electrode 20 is arranged on thesecond piezoelectric layer 14. In FIG. 1A, a voltage applied acrossinput terminals arranged at the first electrode 18 and the sharedelectrode 16 induces an acoustic wave which couples into the secondpiezoelectric layer 14 through the shared electrode 16 forming astanding wave in the full structure. It should be understood that theinput terminals may alternatively be arranged at the second electrode 20and the shared electrode 16. In operation, the SCF 10 exhibits multiplemodes corresponding to the different eigenfunctions the SCF 10 supports.FIG. 1B illustrates a typical transmission response of the SCF 10 ofFIG. 1A exhibiting multiple modes 22-1 to 22-3 formed along thefrequency range. The three main peaks in the transmission responsecorrespond to the modes 22-1 to 22-3 that are excited at progressivelyincreasing frequencies. In this regard, the modes may be referred to asorder modes where a first mode 22-1 is a first order mode, a second mode22-2 is a second order mode, a third mode 22-3 is a third order mode,and so on.

FIGS. 2A-2C respectively illustrate the stress profiles for each of thethree modes 22-1 to 22-3 of FIG. 1B for the SCF 10. For simplicity, verythin electrodes have been used. In FIG. 2A, the stress profile of thefirst mode 22-1 exhibits approximately a half wavelength through the SCF10. In FIG. 2B, the stress profile of the second mode 22-2 exhibitsapproximately a full wavelength through the SCF 10. In FIG. 2C, thestress profile of the third mode 22-3 exhibits approximately one and ahalf wavelength through the SCF 10.

FIG. 3 illustrates an equivalent circuit EC of the SCF 10 about thesecond mode 22-2 of FIG. 2B which is most commonly utilized inapplications. The equivalent circuit EC includes an SCF inductanceL_(SCF) and an SCF capacitance C_(SCF). Shunt capacitances C_(SHU) oneither side represent static capacitances of the piezoelectric layers(12, 14 of FIG. 1A). FIG. 4A illustrates the SCF 10 with shunt inductorsL_(shu) arranged to compensate for such shunt capacitances, and FIG. 4Billustrates an equivalent circuit EC′ of the SCF 10 with the shuntinductors L_(shu) of FIG. 4A. As such, the circuit may behave as aseries resonant circuit about the resonant frequency while exhibiting asymmetric electrical response (S₁₁=S₂₂). Depending on the application,this symmetry may or may not be needed.

In order to understand principles of operation according to embodimentsdisclosed herein, a brief description of relevant theory is provided.FIG. 5 is an illustration of a slab 23 of piezoelectric material 24 witha varying electromechanical coupling along the z-axis. The slab 23 isassumed to be excited with a current I. The governing equations areNewton's second law, and the definition of strain in one dimension.Assuming e^(jωt) time dependency, the equations may be expressedaccording to Equations (1), (2):

$\begin{matrix}{{\frac{dT}{dz} = {{- \omega^{2}}\rho\; u}},} & {(1),} \\{S = \frac{du}{dz}} & (2)\end{matrix}$

Where T and S are the stress and strain respectively, and u is themechanical displacement. ρ is the mass density of the piezoelectricmaterial 24. Since it is assumed that no free charges exist in thepiezoelectric material 24, Gauss law dictates that the derivative of theelectric displacement D,

${\frac{dD}{dz} = {\rho_{v} = 0}},$

or equivalently, D=constant. D is related to the current I by I=jωAD,where A is the area of the slab. A convenient form of piezoelectricconstitutive equations may be expressed according to Equations (3), (4):

S=s ^(D) T+gD,  (3)

E=−gT+β ^(T) D  (4)

which uses T and D as independent variables. Here, g accounts for thecoupling between the electrical and acoustic domains. By combining theabove relations, the second order differential equation can be expressedaccording to Equation (5):

$\begin{matrix}{{\frac{d^{2}T}{{dz}^{2}} + {\omega^{2}s^{D}\rho}} = \frac{j\;{\omega\rho}\;{g(z)}I}{A}} & (5)\end{matrix}$

This is an inhomogeneous equation for the stress T, in which the currentand piezoelectric profile g(z) appear in the source term on theright-hand side of the equation. A solution can be obtained by expandingT in terms of the eigenfunctions of the homogenous equation, whichsatisfy the boundary conditions of zero stress at the piezo-airinterface. These eigenfunctions are the mode profiles the structuresupports (e.g., the SCF 10 of FIGS. 2A-2C). The stress can be expressedaccording to Equation (6):

T(z)=ΣA _(n)(ω)ϕ_(n)(z)  (6)

Utilizing the orthogonality of these eigenfunctions, the amplitude ofexcitation of the different modes can be expressed according to Equation(7):

A _(n)(ω)∝∫g(z)ϕ_(n)(z)dz  (7)

Stated differently, the degree of excitation of any mode in thepiezoelectric material 24 is proportional to the dot product ofpiezoelectric coupling profile with the mode shape. As such, thisproperty may be utilized to provide acoustic resonator configurationsthat suppress the excitation of undesired modes according to embodimentsdisclosed herein.

In certain embodiments, the spurious response of an SCF may besuppressed by modifying the piezoelectric coupling profile within one ormore piezoelectric layers of the SCF, thereby providing an excitationprofile or coupling profile that is orthogonal to one or more spuriousmodes that are to be suppressed. In this regard, multiple SCFs may bearranged in a solidly mounted resonator (SMR) configuration thatutilizes a single reflector stack, thereby simplifying the fabricationof such filters and allowing improved integration with mobilecommunication systems. Additionally, SCFs as disclosed herein may alsobe well suited for incorporation with film bulk acoustic resonator(FBAR) configurations.

FIG. 6 illustrates the stress profile of the third mode 22-3 asillustrated in FIG. 2C where portions of the second piezoelectric layer14 that correspond to net coupling for the third mode 22-3 areindicated. A horizontal dashed line is superimposed to separate thesecond piezoelectric layer 14 into a top portion 14 _(top) and a bottomportion 14 _(bottom). As illustrated, the net coupling from the bottomportion 14 _(bottom) is about zero since there are equal positive andnegative areas in the stress profile. In this regard, net coupling tothe third mode 22-3 is contributed by the top portion 14 _(top) andexcitation of the third mode 22-3 may thereby be suppressed by modifyingthe piezoelectric coupling profile in the top portion 14 _(top).Relative thicknesses of the top portion 14 _(top) and the bottom portion14 _(bottom) can vary with different configurations. In certainembodiments, a relative thickness of the top portion 14 _(top) maycomprise a range including 0.1t to 0.5t where t is the thickness of thesecond piezoelectric layer 14. In certain embodiments, the relativethickness of the top portion 14 _(top) may comprise a range including0.2t to 0.4t. In certain embodiments, the relative thickness of the topportion 14 _(top) may comprise a value of approximately 0.3t with acorresponding value of approximately 0.7t for the bottom portion 14_(bottom). With regard to relative thicknesses, the term “approximately”is defined to mean a nominal thickness within +/−five (5) percent of thevalue. By identifying the portion of the second piezoelectric layer 14responsible for net coupling of the third mode 22-3, modifications tothe piezoelectric coupling profile in the identified portion may beprovided to suppress the third mode 22-3.

FIGS. 7A-7C illustrate SCF configurations that include modifications tothe piezoelectric coupling profile for suppression of the third mode22-3. FIG. 7A illustrates the stress profile of the third mode 22-3 foran SCF 26 where portions of the second piezoelectric layer 14 arereplaced by a non-piezoelectric layer 28. In certain embodiments, thenon-piezoelectric layer 28 is arranged to replace the top portion 14_(top) of the second piezoelectric layer 14 that is responsible for netcoupling of the third mode 22-3 as illustrated in FIG. 6. As such, thenon-piezoelectric layer 28 may comprise a thickness in a range including0.1t to 0.5t, or a range 0.2t to 0.4t, or approximately 0.3t dependingon the embodiment where t is the thickness between the shared electrode16 and the second electrode 20. In certain embodiments, thenon-piezoelectric layer 28 comprises a dielectric layer or a pluralityof dielectric layers, such one or more layers of silicon dioxide (SiO₂),silicon nitride (SiN), or combinations thereof. In such arrangements,the non-piezoelectric layer 28 is generally arranged between the firstelectrode 18 and the second electrode 20 within the SCF 26. Moreparticularly, the non-piezoelectric layer 28 is arranged between thesecond electrode 20 and the second piezoelectric layer 14 in certainembodiments. In this regard, the second piezoelectric layer 14 isconfigured to provide about equal positive and negative areas in thestress profile for suppression of the third mode 22-3. In otherembodiments and depending on the mode to be suppressed, thenon-piezoelectric layer 28 may be arranged in other locations of the SCF26, such as between the first electrode 18 and the first piezoelectriclayer 12.

FIG. 7B illustrates the stress profile of the third mode 22-3 for an SCF30 where the second electrode 20 is arranged with increased thicknessfor suppression of the third mode 22-3. Since the second electrode 20comprises a non-piezoelectric metal layer, the second electrode 20 mayaccordingly be configured to absorb a portion of the stress profile thatcorresponds to the third mode 22-3. In particular, the second electrode20 may be configured with a thickness that is suitable to contain orabsorb the portion of the stress profile that corresponds to the topportion 14 _(top) of the second piezoelectric layer 14 as illustrated inFIG. 6. In this regard, the stress profile of the second piezoelectriclayer 14 may be configured to provide about equal positive and negativeareas for suppression of the third mode 22-3. In certain embodiments,the second electrode 20 is at least fifty percent thicker than the firstelectrode 18. In further embodiments, the second electrode 20 is atleast two times thicker, or at least three times thicker, or at leastfour times thicker than the first electrode 18. In other embodiments anddepending on the mode to be suppressed, the first electrode 18 may bearranged with increased thickness, with or without the second electrode20 having increased thickness.

FIG. 7C illustrates the stress profile of the third mode 22-3 for an SCF32 where an inverted piezoelectric layer is arranged for suppression ofthe third mode 22-3. As illustrated, the SCF 32 comprises a thirdpiezoelectric layer 34 that is arranged between the first electrode 18and the second electrode 20. In certain embodiments, the thirdpiezoelectric layer 34 is arranged between the second electrode 20 andthe second piezoelectric layer 14. In this manner, the thirdpiezoelectric layer 34 may be configured to at least partially replacethe top portion 14 _(top) of the second piezoelectric layer 14 that isresponsible for net coupling of the third mode 22-3 as illustrated inFIG. 6. In certain embodiments, the third piezoelectric layer 34comprises an inverted polarity piezoelectric layer with regard to thesecond piezoelectric layer 14. As such, the second piezoelectric layer14 may have a first polarity and the third piezoelectric layer 34 has asecond polarity that is opposite the first polarity. As illustrated inFIG. 7C, the bottom portion 14 _(bottom) of the second piezoelectriclayer 14 has about zero net coupling. The top portion 14 _(top)contributes a positive coupling and the third piezoelectric layer 34with inverted polarity contributes a compensating negative coupling forsuppression of the third mode 22-3. In certain embodiments, the firstand second polarity assignments may be reversed. That is, the secondpiezoelectric layer 14 (including the top and bottom portions 14 _(top),14 _(bottom)) may have the second polarity and the third piezoelectriclayer 34 may have the first polarity that is inverted from the secondpolarity. In such embodiments, the magnitude response of the SCF 32 maybe the same while providing a phase change (e.g., 180 degrees). Sincethe top and bottom halves of the SCF 32 generally operate separatelyfrom one another, the polarization of the first piezoelectric layer 12can be either the first polarity or the second polarity depending on theapplication. As such, embodiments where the first piezoelectric layer 12comprises the second polarity may provide a phase change (e.g., 180degrees) from embodiments where the first piezoelectric layer 12comprises the first polarity.

In certain embodiments, the first piezoelectric layer 12, the secondpiezoelectric layer 14, and the third piezoelectric layer 34 may eachcomprise aluminum nitride (AlN). For example, the first piezoelectriclayer 12 and the second piezoelectric layer 14 may each comprise anitrogen (N) polar layer of AlN while the third piezoelectric layer 34may comprise an aluminum (Al) polar layer of AlN. In certainembodiments, the second piezoelectric layer 14 and the thirdpiezoelectric layer 34 may be formed by consecutive deposition steps.Said AlN may be undoped or doped with one or more of scandium (Sc),erbium (Er), magnesium (Mg), hafnium (Hf), or the like. In variousembodiments, the first electrode 18, the second electrode 20, and theshared electrode 16 may each comprise one or more metals or metal layerssuch as Al, molybdenum (Mo), tungsten (W), or the like.

In certain embodiments, the third piezoelectric layer 34 may comprise athickness in a range including 0.1t to 0.5t, or a range 0.2t to 0.4t, orapproximately 0.3t, or approximately 0.2t depending on the embodiment,where t is the thickness between the shared electrode 16 and the secondelectrode 20. In other embodiments and depending on the mode to besuppressed, the third piezoelectric layer 34 may be arranged in otherlocations of the SCF 32, such as between the first electrode 18 and thefirst piezoelectric layer 12, or between the shared electrode 16 and thefirst piezoelectric layer 12, or between the shared electrode 16 and thesecond piezoelectric layer 14.

FIG. 8 illustrates a comparison plot for the transmission responses ofthe SCF 32 of FIG. 7C and the SCF 10 of FIG. 1A. In this regard, the SCF10 exhibits three main peaks in the transmission response correspondingto the first three modes 22-1 to 22-3 while the SCF 32 exhibitssuppression of the third mode 22-3. While not shown in the comparisonplot of FIG. 8, the SCF 26 of FIG. 7A and the SCF 30 of FIG. 7B wouldexhibit plots similar to the SCF 32.

A side effect of suppressing the third mode 22-3 for any of theembodiments described above for FIGS. 7A-7C may include reduced couplingto the desired second mode 22-2 near where mode suppression isimplemented. This can result in an asymmetric electrical response whereS₁₁ is not equal to S₂₂. One compensation configuration to restore thesymmetry is to implement mode suppression on both halves or sides of theSCF (e.g., on both the first and second piezoelectric layers); however,this may reduce the coupling to both sides of the SCF. Anothercompensation configuration is to use a higher electromechanical couplingmaterial in or near the piezoelectric material where mode suppression isimplemented. Since the higher electromechanical coupling material willgenerally have different acoustic properties, a different overallthickness of that piezoelectric material may be needed.

FIG. 9 is a diagram illustrating an SCF 36 that includes a secondpiezoelectric layer 38 with a higher electromechanical coupling materialas compensation for the mode suppression of the third mode (e.g., 22-3of FIG. 8). The SCF 36 includes the first electrode 18, the sharedelectrode 16, and the second electrode 20 as previously described. Thefirst piezoelectric layer 12 is arranged between the first electrode 18and the shared electrode 16 and the second piezoelectric layer 38 isarranged between the second electrode 20 and the shared electrode 16.The SCF 36 may include any of the third mode (e.g., 22-3 of FIG. 8)suppression configurations described above for FIGS. 7A-7C. By way ofexample, FIG. 9 includes the third piezoelectric layer 34 as describedfor FIG. 7C. To compensate for any asymmetric electrical response causedby the mode suppression, the second piezoelectric layer 38 may include amaterial with a higher electromechanical coupling than the material ofthe first piezoelectric layer 12. In certain embodiments, the firstpiezoelectric layer 12 and the second piezoelectric layer 38 may eachcomprise AlN with different doping levels to provide different higherelectromechanical coupling. By way of example, the second piezoelectriclayer 38 may comprise a higher doping level (e.g., Sc or the like) thanthe first piezoelectric layer 12. In certain embodiments, the firstpiezoelectric layer 12 may be undoped. Due to the differences inacoustic properties between the first piezoelectric layer 12 and thesecond piezoelectric layer 38, the different halves or sides of the SCF36 as defined on opposing faces of the shared electrode 16 may beconfigured with different thickness (t′ and t in FIG. 9). In FIG. 9, tisthe thickness of the first piezoelectric layer 12 as defined by adistance between the first electrode 18 and the shared electrode 16while t′ is the combined thickness of the third piezoelectric layer 34and the second piezoelectric layer 38 as defined by a distance betweenthe second electrode 20 and the shared electrode 16. In certainembodiments, the thickness t′ of the upper half may configured thinnerthan the thickness t by reducing a thickness of the second piezoelectriclayer 38.

FIGS. 10A-10C are plots comparing differences in electrical responses ofthird mode suppressed SCFs with compensation (e.g., the SCF 36 of FIG.9) and without compensation (e.g., the SCF 32 of FIG. 7C) and a baselineSCF (e.g., the SCF 10 of FIG. 1A). For the purposes of the comparison ofFIGS. 10A and 10B, the top halves of the SCFs 10, 32, 36 are configuredas the SCF input and the bottom halves are configured as the SCF output.Additionally, the results are shown for configurations where the staticshunt capacitances are compensated as illustrated in FIGS. 4A and 4B.FIG. 10A is a Smith chart comparing impedance for a top half or input ofthe SCFs 10, 32, 36 by scattering parameter S₁₁, and FIG. 10B is a Smithchart comparing impedance for a bottom half or output of the SCFs 10,32, 36 by scattering parameter S₂₂. FIG. 10C illustrates a comparisonplot for the transmission responses for the SCFs 10, 32, 36. Asillustrated, the SCF 10 is generally symmetric between the input andoutput of FIGS. 10A and 10B while exhibiting the three modes 22-1 to22-3. For the configuration where the third mode 22-3 is suppressedwithout compensation, the SCF 32 is asymmetric between the input andoutput. As illustrated by the SCF 36, when compensation is added,symmetry may be generally restored while also providing suppression ofthe third mode 22-3.

While the above-described embodiments illustrate various configurationsfor suppression of the third mode, the aspects disclosed may be appliedto suppress any mode or combination of modes in SCFs. In certainembodiments, SCFs may be configured for suppression of the first mode.In other embodiments, SCFs may be configured for suppression of thefirst mode and the third mode. In still other embodiments, SCFs may beconfigured for suppression of the second mode.

FIG. 11 is a stress profile diagram for the first mode 22-1 of an SCF 40where the third piezoelectric layer 34 having inversed polarity isarranged between the first electrode 18 and the shared electrode 16 forsuppression of the first mode 22-1. As with previous embodiments, thethird piezoelectric layer 34 may comprise a thickness in a rangeincluding 0.1t to 0.5t, or a range 0.2t to 0.4t, or approximately 0.3tdepending on the embodiment where t is the thickness between the sharedelectrode 16 and the first electrode 18. By placing the thirdpiezoelectric layer 34 between the first electrode 18 and the sharedelectrode 16, net zero coupling may be exhibited by the bottom half ofthe SCF 40 for suppression of the first mode 22-1. In FIG. 11, the thirdpiezoelectric layer 34 is arranged between the shared electrode 16 andthe first piezoelectric layer 12. In certain embodiments, the thirdpiezoelectric layer 34 may also reduce coupling of other modes, such asthe second mode (22-2 of FIG. 2B) that may be intended as the main modefor the SCF 40. In this regard, the SCF 40 may be compensated byconfiguring different electromechanical coupling materials and/ordifferent thicknesses for the first piezoelectric layer 12 relative tothe second piezoelectric layer 14 as described above. In otherembodiments, the third piezoelectric layer 34 may be provided in otherlocations between the first electrode 18 and the shared electrode 16.

FIG. 12 is a diagram illustrating an SCF 44 that includes modesuppression for the first mode (22-1 of FIG. 2B) and the third mode(22-3 of FIG. 2B). The SCF 44 may include any of the mode suppressionconfigurations described above. By way of example, the SCF 44 includes afourth piezoelectric layer 46 with an inverted polarity that is arrangedbetween the first electrode 18 and the shared electrode 16 forsuppression of the first mode (22-1 of FIG. 2B). The SCF 44 alsoincludes a third piezoelectric layer 48 with an inverted polarity thatis arranged between the second electrode 20 and the shared electrode 16for suppression of the third mode (22-1 of FIG. 2B). To compensate forany asymmetric electrical response caused by the mode suppression, afirst piezoelectric layer 50 that is arranged between the firstelectrode 18 and the shared electrode 16 may include a material with ahigher electromechanical coupling than the material of the secondpiezoelectric layer 14.

Due to the differences in acoustic properties between the firstpiezoelectric layer 50 and the second piezoelectric layer 14, thedifferent halves or sides of the SCF 44 as defined on opposing faces ofthe shared electrode 16 may be configured with different thickness (t′and t in FIG. 12). In FIG. 12, t is the combined thickness of the thirdpiezoelectric layer 48 and the second piezoelectric layer 14 as definedby a distance between the second electrode 20 and the shared electrode16 while t′ is the combined thickness of the fourth piezoelectric layer46 and the first piezoelectric layer 50 as defined by a distance betweenthe first electrode 18 and the shared electrode 16. In certainembodiments, the thickness t′ of the lower half may be configuredthinner than the thickness t by reducing a thickness of the firstpiezoelectric layer 50. In certain embodiments, a thickness of thefourth piezoelectric layer 46 needed to suppress the first mode (22-1 ofFIG. 2B) may be thicker than a thickness of the third piezoelectriclayer 48 needed to suppress the third mode (22-3 of FIG. 2B). By way ofexample, the fourth piezoelectric layer 46 may have a thickness ofapproximately 0.3t′ while the third piezoelectric layer 48 may have athickness of approximately 0.2t where 0.3t′ is greater than 0.2t. Whilethe inverted polarity configuration is illustrated in the upper half ofthe SCF 44 to suppress the third mode (22-3 of FIG. 2B), any suppressionconfiguration described above for FIGS. 7A-7C, includingnon-piezoelectric layers, thicker electrodes, and combinations thereofmay be used.

FIG. 13 illustrates the transmission response for the SCF 44 of FIG. 12.As illustrated, the first mode 22-1 and the third mode 22-3 aresuppressed with the second mode 22-2 serving as a main mode for the SCF44.

FIG. 14 is a diagram illustrating an SCF 52 that includes modesuppression for the second mode (22-2 of FIG. 2B) and the third mode(22-3 of FIG. 2B). The SCF 52 may include any of the mode suppressionconfigurations described above. By way of example, the SCF 52 isconfigured similar to the SCF 44 of FIG. 12, but with a thicker fourthpiezoelectric layer 46 arranged between the shared electrode 16 and thefirst piezoelectric layer 50. By increasing a thickness of the fourthpiezoelectric layer 46 having inverse polarity, the stress profile ofthe second mode 22-2 as illustrated in FIG. 2B may exhibit a net zerocoupling while the stress profile of the first mode 22-1 as illustratedin FIG. 2A will have a positive value for net coupling. As a result, thesecond mode 22-2 may be suppressed while the first mode 22-1 isconfigured as a main mode for the SCF 52. While the inverted polarityconfiguration is illustrated in the upper half of the SCF 52 to suppressthe third mode (22-3 of FIG. 2B), any suppression configurationdescribed above for FIGS. 7A-7C, including non-piezoelectric layers,thicker electrodes, and combinations thereof may be used.

FIG. 15 illustrates the transmission response for the SCF 52 of FIG. 14.As illustrated, the second mode 22-2 and the third mode 22-3 aresuppressed with the first mode 22-1 serving as a main mode for the SCF52. The coupling for the first mode 22-1 may be reduced; however, theabove described compensation method of using different coupling valuesfor the piezoelectric layers may be used.

In certain aspects, various mode suppression configurations can provideelectrically symmetric responses without the need for compensationconfigurations. In certain embodiments, SCFs may be configured with topand bottom halves of differing thicknesses such that stress profiles aredivided differently between the top and bottom halves. In this regard,the top and bottom halves of an SCF may be configured to suppressdifferent modes while also providing equal coupling to input and outputports. With different thicknesses, the top and bottom halves canaccordingly have different shunt capacitances (e.g., FIG. 3), and assuch, different compensating inductors may be provided (e.g., FIGS. 4and 5).

FIG. 16 is a diagram illustrating an SCF 54 that includes modesuppression for the first mode (22-1 of FIG. 2B) and the third mode(22-3 of FIG. 2B) without requiring compensation configurations. The SCF54 includes the fourth piezoelectric layer 46 having inverse polarityand the first piezoelectric layer 12 that collectively define a bottomhalf of the SCF 54. The fourth piezoelectric layer 46 is arrangedbetween the first electrode 18 and the first piezoelectric layer 12 forsuppression of the third mode (22-3 of FIG. 2B). The SCF 54 furtherincludes the third piezoelectric layer 48 having inverse polarity andthe second piezoelectric layer 14 that collectively define a top half ofthe SCF 54. The third piezoelectric layer 48 is arranged between theshared electrode 16 and the second piezoelectric layer 14 forsuppression of the first mode (22-1 of FIG. 2B).

In order to avoid the need for compensation configurations such asdifferent coupling values for one or more of the piezoelectric layers,the top and bottom halves of the SCF 54 are arranged with differentthicknesses. In FIG. 16, T₁ is the combined thickness of the fourthpiezoelectric layer 46 and the first piezoelectric layer 12 as definedby a distance between the first electrode 18 and the shared electrode16, and T₂ is the combined thickness of the third piezoelectric layer 48and the second piezoelectric layer 14 as defined by a distance betweenthe second electrode 20 and the shared electrode 16. As such, a totalthickness T may be defined as the sum of T₁ and T₂. In certainembodiments, T₁ may be configured with a thickness of approximately0.45T, and T₂ may be configured with a thickness of approximately 0.55T.A thickness of the fourth piezoelectric layer 46 as defined between thefirst electrode 18 and the first piezoelectric layer 12 may beapproximately 0.3T₁ for suppression of the third mode. A thickness ofthe third piezoelectric layer 48 as defined between the shared electrode16 and the second piezoelectric layer 14 may be approximately 0.35T₂ forsuppression of the first mode. In this manner, the SCF 54 is configuredto distribute stress differently between the top and bottom halves insuch a way that mode suppression arrangements do not require additionalcompensation. Accordingly, each of the first piezoelectric layer 12 andthe second piezoelectric layer 14 may both comprise the sameelectromechanical coupling material values. While the inverted polarityconfiguration is illustrated in the lower half of the SCF 54 to suppressthe third mode (22-3 of FIG. 2B), any suppression configurationdescribed above for FIGS. 7A-7C, including non-piezoelectric layers,thicker electrodes, and combinations thereof may be used.

FIG. 17A illustrates the transmission response for the SCF 54 of FIG.16. As illustrated, the first mode 22-1 and the third mode 22-3 aresuppressed with the second mode 22-2 serving as a main mode for the SCF54. FIG. 17B is a Smith chart comparing impedance for an input of theSCF 54 of FIG. 16 by scattering parameter S₁₁, and FIG. 17C is a Smithchart comparing impedance for an output of the SCF 54 of FIG. 16 byscattering parameter S₂₂. As illustrated, the input and output of theSCF 54 are generally symmetric.

FIG. 18 is a diagram illustrating an SCF 56 that includes modesuppression for the second mode (22-2 of FIG. 2B) and the third mode(22-3 of FIG. 2B) without requiring compensation configurations. The SCF56 includes the fourth piezoelectric layer 46 having inverse polarityand the first piezoelectric layer 12 that collectively define a bottomhalf of the SCF 56. The fourth piezoelectric layer 46 is arrangedbetween the first electrode 18 and the first piezoelectric layer 12 forsuppression of the third mode (22-3 of FIG. 2B). The SCF 56 furtherincludes the third piezoelectric layer 48 having inverse polarity andthe second piezoelectric layer 14 that collectively define a top half ofthe SCF 54. The third piezoelectric layer 48 is arranged between thesecond electrode 20 and the second piezoelectric layer 14 forsuppression of the second mode (22-2 of FIG. 2B). In FIG. 18, T₁ is thecombined thickness of the fourth piezoelectric layer 46 and the firstpiezoelectric layer 12 as defined by a distance between the firstelectrode 18 and the shared electrode 16, and T₂ is the combinedthickness of the third piezoelectric layer 48 and the secondpiezoelectric layer 14 as defined by a distance between the secondelectrode 20 and the shared electrode 16. As such, a total thickness Tmay be defined as the sum of T₁ and T₂. In certain embodiments, T₁ maybe configured with a thickness of approximately 0.4T, and T₂ may beconfigured with a thickness of approximately 0.6T. A thickness of thefourth piezoelectric layer 46 as defined between the first electrode 18and the first piezoelectric layer 12 may be approximately 0.4T₁ forsuppression of the third mode. A thickness of the third piezoelectriclayer 48 as defined between the second electrode 20 and the secondpiezoelectric layer 14 may be approximately 0.4T₂ for suppression of thesecond mode. In this manner, the SCF 56 is configured to distributestress differently between the top and bottom halves in such a way thatmode suppression arrangements do not require additional compensation.Accordingly, each the first piezoelectric layer 12 and the secondpiezoelectric layer 14 may both comprise the same electromechanicalcoupling material values. While the inverted polarity configuration isillustrated in the lower half of the SCF 56 to suppress the third mode(22-3 of FIG. 2B), any suppression configuration described above forFIGS. 7A-7C, including non-piezoelectric layers, thicker electrodes, andcombinations thereof may be used.

FIG. 19A illustrates the transmission response for the SCF 56 of FIG.18. As illustrated, the second mode 22-2 and the third mode 22-3 aresuppressed with the first mode 22-1 serving as a main mode for the SCF56. FIG. 19B is a Smith chart comparing impedance for an input of theSCF 56 of FIG. 18 by scattering parameter S₁₁, and FIG. 19C is a Smithchart comparing impedance for an output of the SCF 56 of FIG. 18 byscattering parameter S₂₂. As illustrated, the input and output of theSCF 56 are generally symmetric.

In certain embodiments, any of the foregoing aspects, and/or variousseparate aspects and features as described herein, may be combined foradditional advantage. Any of the various features and elements asdisclosed herein may be combined with one or more other disclosedfeatures and elements unless indicated to the contrary herein.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. An acoustic resonator comprising: a firstpiezoelectric layer; a second piezoelectric layer; a shared electrodebetween the first piezoelectric layer and the second piezoelectriclayer; a first electrode on the first piezoelectric layer opposite theshared electrode; a second electrode on the second piezoelectric layeropposite the shared electrode; and a third piezoelectric layer betweenthe second electrode and the shared electrode, the third piezoelectriclayer having a polarity that is opposite a polarity of the secondpiezoelectric layer.
 2. The acoustic resonator of claim 1, wherein thethird piezoelectric layer is between the second electrode and the secondpiezoelectric layer.
 3. The acoustic resonator of claim 1, wherein thethird piezoelectric layer is between the shared electrode and the secondpiezoelectric layer.
 4. The acoustic resonator of claim 1, wherein thesecond piezoelectric layer has a higher electromechanical coupling thanthe first piezoelectric layer.
 5. The acoustic resonator of claim 1,wherein the first piezoelectric layer has a higher electromechanicalcoupling than the second piezoelectric layer.
 6. The acoustic resonatorof claim 1, further comprising a fourth piezoelectric layer between thefirst electrode and the shared electrode, the fourth piezoelectric layerhaving a polarity that is opposite a polarity of the first piezoelectriclayer.
 7. The acoustic resonator of claim 6, wherein the fourthpiezoelectric layer is between the shared electrode and the firstpiezoelectric layer.
 8. The acoustic resonator of claim 7, wherein thethird piezoelectric layer is between the second electrode and the secondpiezoelectric layer.
 9. The acoustic resonator of claim 6, wherein thefourth piezoelectric layer is between the first electrode and the firstpiezoelectric layer.
 10. The acoustic resonator of claim 9, wherein thethird piezoelectric layer is between the second electrode and the secondpiezoelectric layer.
 11. The acoustic resonator of claim 6, wherein thethird piezoelectric layer has a different thickness than the fourthpiezoelectric layer.
 12. The acoustic resonator of claim 6, wherein acombined thickness of the first piezoelectric layer and the fourthpiezoelectric layer is different than a combined thickness of the thirdpiezoelectric layer and the second piezoelectric layer.
 13. The acousticresonator of claim 12, wherein the first piezoelectric layer has adifferent thickness than the second piezoelectric layer.
 14. Theacoustic resonator of claim 1, wherein the first piezoelectric layer hasa different thickness than the second piezoelectric layer.
 15. Anacoustic resonator comprising: a first piezoelectric layer; a secondpiezoelectric layer; a shared electrode between the first piezoelectriclayer and the second piezoelectric layer; a first electrode on the firstpiezoelectric layer opposite the shared electrode; a second electrode onthe second piezoelectric layer opposite the shared electrode; and anon-piezoelectric layer between the first electrode and the secondelectrode.
 16. The acoustic resonator of claim 15, wherein thenon-piezoelectric layer comprises a dielectric layer.
 17. The acousticresonator of claim 15, wherein the dielectric layer comprises silicondioxide or silicon nitride.
 18. The acoustic resonator of claim 15,wherein the non-piezoelectric layer is between the second electrode andthe second piezoelectric layer.
 19. The acoustic resonator of claim 15,wherein the non-piezoelectric layer is between the first electrode andthe first piezoelectric layer.
 20. The acoustic resonator of claim 15,wherein the shared electrode is a metal layer.
 21. An acoustic resonatorcomprising: a first piezoelectric layer; a second piezoelectric layer; ashared electrode between the first piezoelectric layer and the secondpiezoelectric layer; a first electrode on the first piezoelectric layeropposite the shared electrode; and a second electrode on the secondpiezoelectric layer opposite the shared electrode, wherein the secondelectrode is thicker than the first electrode.
 22. The acousticresonator of claim 21, wherein the second electrode is at least fiftypercent thicker than the first electrode.
 23. The acoustic resonator ofclaim 21, wherein the second electrode is at least two times thickerthan the first electrode.